Fuel cell system and method for controlling fuel cell system

Technical field

The present invention relates to a fuel cell system that estimates the current-voltage characteristic of a fuel cell, and to a method of controlling the fuel cell system.

Background art

There is a fuel cell system that estimates the current-voltage (I-V) characteristic of a fuel cell by detecting an output voltage while varying an output current of the fuel cell (see JP 2000-357526A).

Summary of invention

In fuel cell systems that are currently developed, impurities retained in anode gas flow passages in a fuel cell, such as nitrogen gas and water, are discharged to a buffer tank provided in an anode gas discharge passage. Consequently, impurities that are present in electric power generation regions in the fuel cell decrease, and hence a decline in the efficiency of electric power generation by the fuel cell is suppressed.

Such fuel cell systems may be activated in an environment of a subzero temperature. In this case, as the I-V characteristic of the fuel cell worsens compared to the case of normal operation, whether the I-V characteristic of the fuel cell has been restored to the state where electric power can be supplied to a driving motor is regularly checked.

Regarding the estimation of the I-V characteristic, current control is performed to provide the output current extracted from the fuel cell with an amplitude within a certain range, and the current and voltage of the fuel cell are obtained from corresponding sensors sequentially when the output current has the amplitude. Then, the I-V characteristic of the fuel cell is estimated on the basis of the obtained current and voltage values.

Meanwhile, the anode gas pressure is controlled in such a manner that the higher the output current of the fuel cell, the higher the anode gas pressure. In this way, when the output current is low, a stress applied to membranes in the fuel cell due to a pressure difference between the pressure of cathode gas and the anode gas pressure is reduced.

However, in the fuel cell systems that cause impurities to be discharged to the buffer tank, a reduction in the anode gas pressure associated with current control in the I-V estimation causes a backflow of impurities from the buffer tank to the fuel cell, and hence an increase in the concentration of impurities inside the electric power generation regions. As a result, the efficiency of electric power generation by the fuel cell declines, and the voltage of the fuel cell detected by a sensor falls below an intended voltage value. This lowers the precision of estimation of the I-V characteristic.

The present invention has been made in view of the above problems, and aims to provide a fuel cell system that discharges impurities retained in a fuel cell to a container and suppresses a decline in the precision of estimation of the I-V characteristic associated with a backflow of impurities, as well as a method of controlling the fuel cell system.

According to one aspect of the present invention, a fuel cell system is configured to supply anode gas and cathode gas to a fuel cell and cause the fuel cell to generate electric power in accordance with a load. The fuel cell system includes a container configured to reserve impurities discharged from the fuel cell, a pressure control unit configured to make a pressure of the anode gas higher when a current of the fuel cell is high than when the current of the fuel cell is low, and an estimation unit configured to vary the current of the fuel cell, and estimate a current-voltage characteristic of the fuel cell on the basis of a current value and a voltage value obtained when the current of the fuel cell has been varied. The fuel cell system includes a restriction unit configured to, while the estimation unit is estimating the current-voltage characteristic, restrict a reduction made by the pressure control unit in the pressure of the anode gas.

Brief description of drawings

FIG. 1 is a configuration diagram showing a fuel cell system according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a method of estimating the I-V characteristic of fuel cells.

FIG. 3 is a block diagram showing a control unit composing a controller.

FIG. 4 is an explanatory diagram showing a reduction in the output voltage associated with a reduction in the anode gas pressure at the time of I-V estimation.

FIG. 5 is a flowchart showing a method of restricting a reduction in the anode gas pressure at the time of I-V estimation.

FIG. 6 is a block diagram showing an anode gas control unit according to a second embodiment.

FIG. 7 is an explanatory diagram showing a method of restricting a reduction in the anode gas pressure at the time of I-V estimation.

FIG. 8 is an explanatory diagram showing the precision of estimation of the I-V characteristic of the fuel cells.

FIG. 9 is a block diagram showing an anode gas control unit according to a third embodiment.

FIG. 10 is an explanatory diagram showing a method of restricting a reduction in the anode gas pressure at the time of I-V estimation.

Description of embodiments

Embodiments of the present invention will be described below with reference to the drawings and the like.

(First Embodiment)

FIG. 1 shows a configuration of a fuel cell system 100 according to a first embodiment of the present invention.

The fuel cell system 100 is a power supply system that supplies cathode gas and anode gas from outside to a fuel cell stack 1 , and causes the fuel cell stack 1 to generate electric power in accordance with loads. In the present embodiment, the fuel cell system 100 supplies the electric power generated by the fuel cell stack 1 to the loads, such as a driving motor 53 .

The fuel cell system 100 includes the fuel cell stack 1 , a cathode gas supply/discharge apparatus 2 , an anode gas supply/discharge apparatus 3 , a stack cooling apparatus 4 , an electric power system 5 , and a controller 6 .

The fuel cell stack 1 is a stacked battery made by stacking several hundred fuel cells (so-called battery cells). The fuel cell stack 1 generates electric power using anode gas and cathode gas supplied thereto. The fuel cell stack 1 has an anode electrode output terminal 11 and a cathode electrode output terminal 12 as terminals via which the electric power is extracted.

Each fuel cell is composed of an anode electrode (fuel electrode), a cathode electrode (oxidant electrode), and an electrolyte membrane interposed between the anode electrode and the cathode electrode. In each fuel cell, anode gas (fuel gas) containing hydrogen at the anode electrode, and cathode gas (oxidant gas) containing oxygen at the cathode electrode, bring about an electrochemical reaction at the electrolyte membrane. The following electrochemical reactions proceed in the anode electrode and the cathode electrode.

Anode electrode: 2H.sub.2.fwdarw.4H.sup.++4e.sup.−

Cathode electrode: 4H.sup.++4e.sup.−+O.sub.2.fwdarw.2H.sub.2O

In each fuel cell, the foregoing electrochemical reactions

and

generate an electromotive force and water. The fuel cells stacked in the fuel cell stack 1 are connected in series. Thus, the sum of cell voltages generated in the fuel cells is the output voltage of the fuel cell stack 1 (e.g., several hundred volts).

The cathode gas supply/discharge apparatus 2 and the anode gas supply/discharge apparatus 3 supply cathode gas and anode gas, respectively, to the fuel cell stack 1 .

The cathode gas supply/discharge apparatus 2 supplies cathode gas to the fuel cell stack 1 , and discharges cathode off-gas discharged from the fuel cell stack 1 to the outer air. The cathode gas supply/ discharge apparatus 2 includes a cathode gas supply passage 21 , a filter 22 , a cathode compressor 23 , a cathode pressure sensor 24 , a cathode gas discharge passage 25 , and a cathode pressure regulator valve 26 .

Cathode gas to be supplied to the fuel cell stack 1 flows in the cathode gas supply passage 21 . The cathode gas supply passage 21 is connected to the filter 22 at one end, and to a cathode gas inlet hole of the fuel cell stack 1 at the other end.

The filter 22 removes foreign substances in cathode gas to be introduced into the cathode gas supply passage 21 .

The cathode compressor 23 is provided in the cathode gas supply passage 21 . The cathode compressor 23 introduces the air (outer air) as cathode gas into the cathode gas supply passage 21 via the filter 22 , and supplies that air to the fuel cell stack 1 .

The cathode pressure sensor 24 is provided in the cathode gas supply passage 21 so as to be located downstream relative to the cathode compressor 23 . The cathode pressure sensor 24 detects the pressure of cathode gas to be supplied to the fuel cell stack 1 . The pressure detected by the cathode pressure sensor 24 is output to the controller 6 .

Cathode off-gas that has been discharged from the fuel cell stack 1 flows in the cathode gas discharge passage 25 . The cathode gas discharge passage 25 is connected to a cathode gas outlet hole of the fuel cell stack 1 at one end, and is open at the other end.

The cathode pressure regulator valve 26 is provided in the cathode gas discharge passage 25 . The cathode pressure regulator valve 26 is controlled to open/close by the controller 6 , and adjusts the pressure of cathode gas supplied from the cathode compressor 23 to the fuel cell stack 1 to a desired pressure.

Although not shown in FIG. 1 , a humidifier apparatus may be provided in the cathode gas supply passage 21 to humidify the fuel cell stack 1 .

The anode gas supply/discharge apparatus 3 , which is a dead-end system, supplies anode gas to the fuel cell stack 1 and discharges, to the cathode gas discharge passage 25 , anode off-gas that has been discharged from the fuel cell stack 1 . The anode gas supply/discharge apparatus 3 includes a high-pressure tank 31 , an anode gas supply passage 32 , an anode pressure regulator valve 33 , an anode pressure sensor 34 , an anode gas discharge passage 35 , a buffer tank 36 , and a purge valve 37 .

The high-pressure tank 31 reserves anode gas to be supplied to the fuel cell stack 1 while maintaining the same in a high-pressure state.

Anode gas is supplied from the high-pressure tank 31 to the fuel cell stack 1 via the anode gas supply passage 32 . The anode gas supply passage 32 is connected to the high-pressure tank 31 at one end, and to an anode gas inlet hole of the fuel cell stack 1 at the other end.

The anode pressure regulator valve 33 is provided in the anode gas supply passage 32 . The anode pressure regulator valve 33 is controlled to open/close by the controller 6 , and adjusts the pressure of anode gas that has flowed from the high-pressure tank 31 to the anode gas supply passage 32 to a desired pressure.

The anode pressure sensor 34 is provided in the anode gas supply passage 32 so as to be located downstream relative to the anode pressure regulator valve 33 . The anode pressure sensor 34 detects the pressure of anode gas to be supplied to the fuel cell stack 1 . The pressure detected by the anode pressure sensor 34 is output to the controller 6 .

Anode off-gas that has been discharged from the fuel cell stack 1 flows in the anode gas discharge passage 35 . The anode gas discharge passage 35 is connected to an anode gas outlet hole of the fuel cell stack 1 at one end, and to the cathode gas discharge passage 25 at the other end.

The buffer tank 36 is provided in the anode gas discharge passage 35 . The buffer tank 36 is a container that reserves anode off-gas that has been discharged from the fuel cell stack 1 . The buffer tank 36 allows impurity gases to be discharged to a region that is downstream relative to electric power generation regions in the fuel cell stack 1 . This can suppress an increase in the concentration of impurity gases in the electric power generation regions in the fuel cell stack 1 .

Note that an electric power generation region in the fuel cell stack 1 denotes a region where the electrolyte membrane is interposed between an anode gas flow passage and a cathode gas flow passage in each fuel cell. In place of the buffer tank 36 , a container may be provided in a portion where the anode gas flow passages in the fuel cells merge inside the fuel cell stack 1 so as to accumulate anode off-gas therein.

The purge valve 37 is provided in the anode gas discharge passage 35 . The purge valve 37 is controlled to open/close by the controller 6 , and controls the flow rate of anode off-gas discharged from the anode gas discharge passage 35 to the cathode gas discharge passage 25 .

The stack cooling apparatus 4 cools the fuel cell stack 1 , and maintains the fuel cell stack 1 at a temperature appropriate for the electric power generation. The stack cooling apparatus 4 includes a cooling water circulation passage 41 , a radiator 42 , a bypass passage 43 , a three-way valve 44 , a circulation pump 45 , a PTC heater 46 , a first water temperature sensor 47 , and a second water temperature sensor 48 .

Cooling water for cooling the fuel cell stack 1 circulates in the cooling water circulation passage 41 .

The radiator 42 is provided in the cooling water circulation passage 41 , and cools the cooling water that has been discharged from the fuel cell stack 1 .

The bypass passage 43 allows cooling water to bypass the radiator 42 so as to circulate the same to the fuel cell stack 1 . The bypass passage 43 is connected to the cooling water circulation passage 41 at one end, and to the three-way valve 44 at the other end.

The three-way valve 44 is provided in the cooling water circulation passage 41 so as to be located downstream relative to the radiator 42 . The three-way valve 44 switches between circulation paths of cooling water in accordance with the temperature of cooling water.

The circulation pump 45 is provided in the cooling water circulation passage 41 so as to be located downstream relative to the three-way valve 44 , and circulates cooling water.

The PTC heater 46 is provided in the bypass passage 43 . Electric power is applied to the PTC heater 46 at the time of warm-up of the fuel cell stack 1 so as to increase the temperature of cooling water.

The first water temperature sensor 47 is provided in the cooling water circulation passage 41 so as to be located upstream relative to the radiator 42 . The first water temperature sensor 47 detects the temperature of cooling water that has been discharged from the fuel cell stack 1 (hereinafter referred to as “stack outlet water temperature”).

The second water temperature sensor 48 is provided in the cooling water circulation passage 41 so as to be located between the circulation pump 45 and the fuel cell stack 1 . The second water temperature sensor 48 detects the temperature of cooling water to be supplied to the fuel cell stack 1 .

The electric power system 5 includes a current sensor 51 , a voltage sensor 52 , the driving motor 53 , an inverter 54 , a battery 55 , a DC/DC converter 56 , and auxiliary machines 57 .

The current sensor 51 detects the current extracted from the fuel cell stack 1 (hereinafter referred to as “output current”). The output current detected by the current sensor 51 is output to the controller 6 .

The voltage sensor 52 detects a potential difference between the potential of the anode electrode output terminal 11 and the potential of the cathode electrode output terminal 12 (hereinafter referred to as “output voltage”). The output voltage detected by the voltage sensor 52 is output to the controller 6 .

The driving motor 53 is a three-phase alternating-current synchronous motor including a rotor with an embedded permanent magnet, and a stator around which stator coils are wound. The driving motor 53 has functions of an electric motor that is rotated and driven by receiving the electric power supplied from the fuel cell stack 1 and the battery 55 , as well as functions of an electric generator that generates an electromotive force at both ends of the stator coils by rotating the rotor using an external force when a vehicle decelerates.

The inverter 54 is composed of, for example, a plurality of semiconductor switches, such as insulated-gate bipolar transistors (IGBTs). The semiconductor switches of the inverter 54 are controlled to turn on/off by the controller 6 . Consequently, direct-current electric power is converted into alternating-current electric power, or vice versa.

In order to cause the driving motor 53 to function as the electric motor, the inverter 54 converts the direct-current electric power generated by combining the electric power generated by the fuel cell stack 1 and the output electric power of the battery 55 into three-phase alternating-current electric power, and supplies the three-phase alternating-current electric power to the driving motor 53 . On the other hand, in order to cause the driving motor 53 to function as the electric generator, the inverter 54 converts the regenerative electric power (three-phase alternating-current electric power) of the driving motor 53 into direct-current electric power, and supplies the direct-current electric power to the battery 55 .

The battery 55 is charged with the regenerative electric power of the driving motor 53 or the electric power generated by the fuel cell stack 1 . The electric power with which the battery 55 has been charged is supplied to the auxiliary machines 57 and the driving motor 53 as needed.

The DC/DC converter 56 is a bidirectional voltage converter that increases/reduces the output voltage of the fuel cell stack 1 . One voltage terminal of the DC/DC converter 56 is connected to the fuel cell stack 1 , whereas the other voltage terminal of the DC/DC converter 56 is connected to the battery 55 .

The DC/DC converter 56 increases or reduces the voltage generated at its voltage terminal connected to the fuel cell stack 1 using the electric power of the battery 55 . The DC/DC converter 56 adjusts the output voltage of the fuel cell stack 1 , and controls the output current of the fuel cell stack 1 and the electric power generated by the fuel cell stack 1 (output current x output voltage).

The auxiliary machines 57 are connected in parallel between the DC/DC converter 56 and the battery 55 . The auxiliary machines 57 include the cathode compressor 23 , the circulation pump 45 , the PTC heater 46 , and the like, and are driven by receiving the electric power supplied from the battery 55 or the fuel cell stack 1 .

The controller 6 is composed of a microcomputer provided with a central processing unit (CPU), a read-only memory (ROM), a random-access memory (RAM), and an input/output (I/O) interface.

The controller 6 receives, as input, signals from the aforementioned first water temperature sensor 47 , second water temperature sensor 48 , current sensor 51 , and voltage sensor 52 . The controller 6 also receives, as input, signals from various types of sensors that are necessary for controlling the fuel cell system 100 .

Other sensors include a key sensor 61 and an acceleration stroke sensor 62 . The key sensor 61 detects a start request and a stop request for the fuel cell system 100 on the basis of on/off of a start key. The acceleration stroke sensor 62 detects a depression amount of an accelerator pedal.

Upon receiving a start request from the key sensor 61 , the controller 6 determines that the fuel cell system 100 has been activated, and performs warm-up control to warm up the fuel cell stack 1 to a temperature appropriate for the electric power generation (hereinafter referred to as “warm-up facilitation operation”).

In the warm-up facilitation operation, the controller 6 supplies electric power from the fuel cell stack 1 to the auxiliary machines 57 by controlling the DC-to-CD converter 56 , thereby causing the fuel cell stack 1 to generate electric power necessary for driving the auxiliary machines 57 . As the fuel cell stack 1 generates heat by itself through the electric power generation, the fuel cell stack 1 warms up by itself.

Furthermore, during the warm-up facilitation operation, the controller 6 sets the rotation speed of the circulation pump 45 to an upper limit value of a variable range, and sets the output of the PTC heater 46 , that is to say, the amount of heat generation by the PTC heater 46 to an upper limit value of a variable range.

Consequently, the fuel cell stack 1 is warmed up also by cooling water that has been heated by the PTC heater 46 . In addition, as the electric power consumed by the circulation pump 45 and the PTC heater 46 increases, the electric power generated by the fuel cell stack 1 increases, and the amount of self-heat generation by the fuel cell stack 1 increases as well. Thus, warm-up of the fuel cell stack 1 is further facilitated.

By performing the foregoing warm-up facilitation operation, a warm-up period from the activation of the fuel cell system 100 to the completion of warm-up of the fuel cell stack 1 can be shortened.

Furthermore, when the fuel cell stack 1 is activated, the controller 6 executes computation processing to estimate the current-voltage (I-V) characteristic of the fuel cell stack 1 (hereinafter referred to as “I-V estimation”). Now, the I-V estimation will be briefly described with reference to FIG. 2 .

FIG. 2 shows a relationship between the I-V characteristic of the fuel cell stack 1 and the temperature of the fuel cell stack 1 .

As shown in FIG. 2 , with a reduction in the temperature of the fuel cell stack 1 , the I-V characteristic of the fuel cell stack 1 falls below a reference I-V characteristic, and the electric power generated by the fuel cell stack 1 (voltage x current) decreases. The reference I-V characteristic is prescribed on the basis of, for example, the I-V characteristic during normal operation.

In the case of the I-V characteristic indicated by a dash line in FIG. 2 , current A is the output current with which the fuel cell stack 1 generates the minimum driving electric power, and voltage V 1 is the output voltage with which the output current A is extracted. The minimum driving electric power is a lower limit value of electric power that allows the driving motor 53 to drive the vehicle.

In this case, as the minimum driving electric power can be supplied from the fuel cell stack 1 to the driving motor 53 , the controller 6 permits the vehicle to run so as to enable a supply of electric power from the fuel cell stack 1 to the driving motor 53 .

On the other hand, in the case of the I-V characteristic indicated by a dash-and-dot line in FIG. 2 , voltage V 2 is the output voltage with which the output current A is extracted from the fuel cell stack 1 . In the case of this I-V characteristic, if the output voltage of the fuel cell stack 1 is set to a voltage value within a range that allows both of the driving motor 53 and the inverter 54 to operate, then the output current extracted from the fuel cell stack 1 decreases, and hence the electric power generated by the fuel cell stack 1 falls below the minimum driving electric power.

In this case, as the minimum driving electric power cannot be supplied from the fuel cell stack 1 to the driving motor 53 , the controller 6 prohibits a supply of electric power from the fuel cell stack 1 to the driving motor 53 .

In order to quickly permit the vehicle to run, it is necessary to accurately determine that the fuel cell stack 1 generates electric power equal to or higher than the minimum driving electric power when the output voltage of the fuel cell stack 1 is set to a voltage value that allows the driving motor 53 and the inverter 54 to operate.

Next, a method of estimating the I-V characteristic of the fuel cell stack 1 will be briefly described.

It is known that the relationship between the output current I and a difference ΔV obtained by subtracting the actual output voltage (detected value) from a reference voltage specified by the reference I-V characteristic can be approximated using a linear function indicated by expression

on the condition that the influence of concentration overpotential is small. Δ V=aI+b

The controller 6 performs current control to vary the output current of the fuel cell stack 1 with a predetermined amplitude, and measures the output current and the output voltage (electric power generation voltage) multiple times using the current sensor 51 and the voltage sensor 52 while varying the output current. The controller 6 calculates “a” and “b” in expression

from the measured output currents and output voltages.

Once calculated, “a” and “b” in expression

yield the output voltage corresponding to the output current A with which the minimum driving electric power is supplied from the fuel cell stack 1 to the driving motor 53 . If the yielded output voltage is equal to or higher than the voltage V 1 , then the fuel cell stack 1 can supply the minimum driving electric power to the driving motor 53 .

The controller 6 repeatedly performs the I-V estimation at a predetermined interval (e.g., at a five-second interval) from when the I-V estimation is started until when the fuel cell stack 1 can supply the minimum driving electric power to the driving motor 53 .

A description is now given of a functional configuration of the controller 6 .

FIG. 3 is a functional block diagram showing an example of a control unit 200 composing the controller 6 .

The control unit 200 causes the fuel cell stack 1 to generate electric power on the basis of input signals from various types of sensors provided in the fuel cell system 100 and instruction values for control devices and the like.

The control unit 200 controls the cathode compressor 23 , the cathode pressure regulator valve 26 , the anode pressure regulator valve 33 , and the purge valve 37 to supply anode gas and cathode gas to the fuel cell stack 1 at a flow rate appropriate for the electric power generation. After the vehicle has been permitted to run, the control unit 200 controls the DC/DC converter 56 to supply the electric power generated by the fuel cell stack 1 to the inverter 54 and the auxiliary machines 57 .

The control unit 200 also performs the warm-up facilitation operation. Specifically, when the fuel cell system 100 is activated, the control unit 200 determines whether the temperature of the fuel cell stack 1 is lower than a predetermined warm-up threshold (e.g., 50° C.). For example, a stack inlet water temperature or an average of the stack inlet water temperature and the stack outlet water temperature is used as the temperature of the fuel cell stack 1 .

If it is determined that the temperature of the fuel cell stack 1 is lower than the warm-up threshold, the control unit 200 starts the warm-up facilitation operation to increase the electric powers consumed by the auxiliary machines 57 and extract a predetermined current necessary for warm-up (hereinafter referred to as “current required for warm-up”) from the fuel cell stack 1 . For example, the control unit 200 sets each of the electric powers consumed by the cathode compressor 23 , the circulation pump 45 , and the PTC heater 46 included among the auxiliary machines 57 to an upper limit value of a variable range.

When the temperature of the fuel cell stack 1 has increased to the warm-up threshold through the warm-up facilitation operation, the control unit 200 determines that warm-up of the fuel cell stack 1 has been completed, and ends the warm-up facilitation operation.

On the other hand, if it is determined that the temperature of the fuel cell stack 1 is higher than the warm-up threshold, normal operation is performed to control the pressures of anode gas and cathode gas in accordance with the electric power required by the driving motor 53 .

On the basis of the electric power required by the auxiliary machines 57 and the driving motor 53 , the control unit 200 calculates a target value of the current extracted from the fuel cell stack 1 (hereinafter referred to as “target current”). The larger the depression amount detected by the acceleration stroke sensor 62 , the higher the target current.

Once the control unit 200 has calculated the target current, the control unit 200 calculates a target value of the pressure of anode gas to be supplied to the fuel cell stack 1 (target pressure) on the basis of the target current. Specifically, the higher the target current, the higher the target pressure of anode gas set by the control unit 200 . The control unit 200 also calculates target values of the pressure and flow rate of cathode gas to be supplied to the fuel cell stack 1 on the basis of the target current.

Once the control unit 200 has calculated the target current, the control unit 200 refers to the I-V characteristic of the fuel cell stack 1 to calculate a voltage value corresponding to the target current as a target voltage. Then, the control unit 200 adjusts the voltage terminal of the DC/DC converter 56 connected to the fuel cell stack 1 to the target voltage. Consequently, the fuel cell stack 1 outputs current equivalent to the target current.

The control unit 200 includes a pressure control unit 210 , an I-V estimation unit 220 , and an anode gas restriction unit 230 .

The pressure control unit 210 controls the cathode compressor 23 and the cathode pressure regulator valve 26 to make the pressure of cathode gas to be supplied higher when the target current of the fuel cell stack 1 is high than when the target current of the fuel cell stack 1 is low.

Similarly, the pressure control unit 210 increases the duty cycle of the anode pressure regulator valve 33 to make the pressure of anode gas to be supplied to the fuel cell stack 1 higher when the target current of the fuel cell stack 1 is high than when the target current of the fuel cell stack 1 is low.

For example, the pressure control unit 210 stores a pressure map indicating correspondence between power generation currents and anode gas pressures. Upon obtaining the target current, the pressure control unit 210 calculates an anode gas pressure corresponding to the target current as the target pressure. The anode pressure regulator valve 33 adjusts the pressure of anode gas to be supplied to the fuel cell stack 1 to the target pressure.

As such, the pressure of supplied anode gas is increased in accordance with the output current of the fuel cell stack 1 . Therefore, when the output current is low, the pressure of supplied anode gas decreases. Accordingly, an interelectrode pressure difference between the anode gas pressure and the cathode gas pressure inside the fuel cell stack 1 can be suppressed. As a result, a stress applied to the electrolyte membranes inside the fuel cell stack 1 can be suppressed.

In order to estimate the I-V characteristic of the fuel cell stack 1 , the I-V estimation unit 220 performs current control to vary the output current of the fuel cell stack 1 so as to have an amplitude. In the current control, the I-V estimation unit 220 first increases the target current from the current required for warm-up to an upper limit value for the I-V estimation, and then reduces the target current to the current required for warm-up.

When the output current of the fuel cell stack 1 has been varied, the I-V estimation unit 220 obtains a current value and a voltage value from the current sensor 51 and the voltage sensor 52 sequentially. The I-V estimation unit 220 computes a straight approximation line that enables estimation of the I-V characteristic of the fuel cell stack 1 on the basis of expression

by using, among these current values and voltage values, the current values and voltage values obtained while reducing the output current of the fuel cell stack 1 .

In the foregoing manner, the I-V estimation unit 220 successively estimates the I-V characteristic of the fuel cell stack 1 . The I-V estimation is repeated until the I-V characteristic is restored to the state where the minimum driving electric power can be supplied from the fuel cell stack 1 to the driving motor 53 .

In the aforementioned fuel cell system 100 , when the I-V estimation unit 220 reduces the target current of the fuel cell stack 1 after increasing the same, the pressure control unit 210 reduces the target pressure of anode gas. As a result, the pressure of anode gas to be supplied to the fuel cell stack 1 decreases. Thus, impurities accumulated in the buffer tank 36 flow back to the fuel cell stack 1 , the concentration of impurities in the electric power generation regions in the fuel cell stack 1 increases, and the efficiency of electric power generation by the fuel cell stack 1 declines.

FIG. 4 is an explanatory diagram showing a reduction in the output voltage of the fuel cell stack 1 associated with ordinary I-V estimation. In FIG. 4 , in order to eject generated water and nitrogen gas that are retained in a downstream side of the anode gas flow passages formed inside the fuel cell stack 1 to the buffer tank 36 , the controller 6 performs a pulsation operation of causing the anode gas pressure to pulsate by controlling the opening degree of the anode pressure regulator valve 33 .

FIG. 4(A) shows fluctuations in the pressure of anode gas to be supplied to the fuel cell stack 1 . In FIG. 4(A) , the pulsated pressure of anode gas during the I-V estimation is indicated by a solid line, and a pulsation upper limit pressure and a pulsation lower limit pressure of anode gas are indicated by dash lines.

FIG. 4(B) shows the output current extracted from the fuel cell stack 1 . FIG. 4(C) shows the output voltage of the fuel cell stack 1 . Horizontal axes in FIGS. 4(A) to 4(C) represent a common time axis.

Immediately before time t 41 , the warm-up facilitation operation is performed, and current is supplied from the fuel cell stack 1 only to the auxiliary machines 57 under voltage control by the DC/DC converter 56 . Furthermore, due to the pulsation operation on anode gas, impurities retained in the fuel cell stack 1 are ejected to and accumulated in the buffer tank 36 .

In an I-V estimation period from time t 41 to time t 43 , the I-V estimation unit 220 performs current control.

Specifically, at time t 41 , as shown in FIG. 4(B) , the output current of the fuel cell stack 1 increases from the current required for warm-up under current control by the I-V estimation unit 220 . The output current corresponding to the increase is supplied from the fuel cell stack 1 to the battery 55 via the DC/DC converter 56 . Note that the current required for warm-up is a current value extracted from the fuel cell stack 1 during the warm-up facilitation operation.

With the increase in the output current of the fuel cell stack 1 , the pulsation lower limit pressure and the pulsation upper limit pressure both increase while the pulsation amplitude defined by the pulsation lower limit pressure and the pulsation upper limit pressure is maintained constant by the pressure control unit 210 , as shown in FIG. 4(A) .

In order to increase the output current of the fuel cell stack 1 , the output voltage of the fuel cell stack 1 is reduced by applying voltage control to the DC/DC converter 56 in accordance with the I-V characteristic of the fuel cell stack 1 , as shown in FIG. 4(C) .

At time t 42 , the output current of the fuel cell stack 1 reaches an upper limit value set by the I-V estimation unit 220 . Subsequently, the output current is reduced to the current required for warm-up under current control by the I-V estimation unit 220 , as shown in FIG. 4(B) . Note that the upper limit value of the output current is set beforehand on the basis of, for example, the amount of current that can be supplied to the battery 55 .

While reducing the output current of the fuel cell stack 1 , the I-V estimation unit 220 sequentially obtains, from the current sensor 51 and the voltage sensor 52 , the output current and the output voltage of the fuel cell stack 1 that are detected at a predetermined sampling interval.

With the reduction in the output current of the fuel cell stack 1 , the pulsation lower limit pressure and the pulsation upper limit pressure are both reduced by the pressure control unit 210 , as shown in FIG. 4(A) .

Therefore, in a current reduction period from time t 42 to time t 43 , the pulsation lower limit pressure decreases while reducing the pulsated pressure of anode gas. At this time, the opening degree of the anode pressure regulator valve 33 is controlled so that the amount of anode gas consumed by the fuel cell stack 1 exceeds the flow rate of supplied anode gas. As a result, a reduction in the pulsated pressure of anode gas exceeds a predetermined pulsation amplitude.

When the amplitude corresponding to a reduction in the pulsated pressure of anode gas increases, the anode gas pressure inside the fuel cell stack 1 temporarily falls below the internal pressure the buffer tank 36 . This causes impurities accumulated in the buffer tank 36 to flow back to the fuel cell stack 1 . Consequently, the concentration of impurities inside the fuel cell stack 1 increases, and the efficiency of electric power generation by the fuel cell stack 1 declines temporarily.

Thus, as shown in FIG. 4(C) , the output voltage of the fuel cell stack 1 falls below an intended voltage value indicated by a dash line. It is under such a circumstance that the I-V characteristic of the fuel cell stack 1 is estimated on the basis of the output voltage detected by the voltage sensor 52 and the output current detected by the current sensor 51 . This lowers the precision of estimation of the I-V characteristic of the fuel cell stack 1 .

Note that in a current reduction period from time t 45 to time t 46 , the amplitude corresponding to a reduction in the pulsated pressure of anode gas does not exceed the predetermined pulsation amplitude as shown in FIG. 4(A) , and thus there is no backflow of impurities from the buffer tank 36 . Accordingly, as shown in FIG. 4(C) , the output voltage of the fuel cell stack 1 does not undergo a voltage reduction associated with a backflow of impurities, and matches an intended voltage value. However, at time t 47 , the output voltage of the fuel cell stack 1 temporarily decreases as shown in FIG. 4(C) because the amplitude corresponding to a reduction in the pulsated pressure exceeds the predetermined pulsation amplitude as shown in FIG. 4(A) .

As such, in the ordinary fuel cell system 100 , the anode gas pressure undergoes a reduction associated with control for reducing the target current in the I-V estimation period. This causes a backflow of impurities from the buffer tank 36 to the fuel cell stack 1 . As a result, the concentration of impurities inside the electric power generation region in each battery cell increases. Accordingly, the overall efficiency of electric power generation by the fuel cell stack 1 declines, and the output voltage of the fuel cell stack 1 falls below an intended voltage value. This lowers the precision of estimation of the I-V characteristic of the fuel cell stack 1 .

To address the foregoing issue, in the present embodiment, the anode gas restriction unit 230 is provided in the control unit 200 shown in FIG. 3 .

The anode gas restriction unit 230 is a restriction unit that, on the basis of a set value of an I-V estimation flag, sets the target pressure of anode gas to a predetermined lower limit value for preventing a backflow of impurities from the buffer tank 36 to the fuel cell stack 1 (hereinafter referred to as “backflow prevention lower limit value”). The backflow prevention lower limit value is set beforehand on the basis of, for example, data of experiments and the like. Alternatively, the backflow prevention lower limit pressure may be set on the basis of the speed of a reduction in the anode gas pressure and the amplitude corresponding to a reduction in the anode gas pressure.

The description continues in the full USPTO document.